(1) Field of the Invention
The present invention relates to a reactive extrusion process for the bulk polymerization of a cyclic aliphatic ester monomer, using an organometallic polymerization agent (catalyst or initiator), particularly a coordination insertion catalyst, to form a high molecular weight polymer with good control over monomer conversion and molecular weight for use as biodegradable articles. Further, the present invention relates to a reactive extrusion process for the bulk polymerization of a lactone, specifically .epsilon.-caprolactone, using coordination insertion catalysts to form a high molecular weight polymer. The polymerization process can optionally be conducted in the presence of other components that bear functionalities like hydroxyl and amino groups including polysaccharides like starches and cellulosics and other types of polymers, which further can be grafted to the polymerized ester.
More specifically, the present invention relates to a continuous extrusion polymerization process to produce high molecular weight poly(.epsilon.-caprolactone) compositions at high throughputs. The present invention also relates to compositions derived by the reactive extrusion polymerization process, which are useful in making biodegradable articles and specifically relates to a unique polymer intermediate that exhibits branching. The present invention also relates to compositions derived from admixtures and blends of the polymer intermediate with other components, especially starch.
(2) Description of Related Art
The solution polymerization of .epsilon.-caprolactone has been the subject of a number of patents over the last few decades. Growing environmental concerns and unfavorable economics related to the use of organic solvents, make bulk polymerization (without the use of solvents) an attractive alternative for polymer synthesis. Further, bulk polymerization can be conducted in extruders, making it a continuous process. Polystyrenes and nylons have been produced commercially by polymerization in an extruder. A number of patents have evolved regarding polymerization based on variations in extruder screw configurations and polymerization mechanisms. Since different polymerization mechanisms are involved in the extrusion of these polymers, the process schematics and screw configurations vary considerably. The extruder screw configuration can have an effect on yield, molecular weight, molecular weight distribution and product throughput.
Reactive extrusion is an attractive route for polymer processing in order to carry out various reactions including polymerization, grafting, branching and functionalization. Reactive extrusion polymerization involves polymerizing a liquid/solid monomer or pre-polymer within the residence time available in the extruder to form a high molecular weight melt.
The prior art has shown that extruders can be used for bulk polymerizations of monomers like methylmethacrylate, styrene, lactam, and lactide (Michaeli, W., et al., J. of Appl. Polymer Sci. 48:871-886 (1993); Kye, H., et al., J. of Appl. Polymer Sci. 52:1249-1262 (1994); and U.S. Pat. No. 5,412,005). The economics of using the extruder as a bulk polymerization reactor are favorable when high throughputs and control of molecular weight are realized. This places a limitation on the residence time required to complete the polymerization, which ideally should be less than 5 minutes.
Bulk polymerization of .epsilon.-caprolactone using coordination insertion catalysts has been discussed by Young et al, ACS Symposium Ser., 59, pages 152-164 (1977). Bulk polymerization of .epsilon.-caprolactone in an extruder in the presence of starch to give a compatibilized blend of polycaprolactone, starch and grafted starch-g-polycaprolactone is described in Narayan et al., U.S. patent application Ser. No. 08/400,979, which is incorporated by reference. EP 626405 describes the preparation of low molecular weight polycaprolactone polymer of number average molecular weight, M.sub.n, ranging from 35,000 to 66,000 at residence times ranging from 6 to 15 minutes respectively. It is believed that higher molecular weight will improve film processing and enhance properties. Furthermore, a residence time of 15 minutes makes the process economically unattractive.
Starch graft copolymers produced from various monomers, including styrene, methyl methacrylate, methyl acrylate, and butyl acrylate, containing about 50% starch by weight have been prepared by a solution process in which the starch grafting was initiated by radiation in the case of styrene and by cerium ion in the case of other monomers (Bagley, Fanta et. al., Polymer Engineering and Science, 17 (5), p. 311 (1977)). These compositions were extruded directly without addition of plasticizer of homopolymer to give useful products. However, the reaction times for polymerizations were on the order of hours. Starch-acrylamide copolymers were prepared by Ce.sup.4+ initiated grafting reactions in solution (Pledger, Young et. al., J. Macromol. Sci.-Chem. A22(4), p. 415 (1986)). Anionic polymerization of ethylene oxide on starch has also been reported (Tahan and Zilkha, Journal of Polymer Science: A-1, 7, p. 1815 (1969)). Reactive extrusion of starch graft copolymers using starch macroradicals generated by shear inside an extruder in the presence of vinylic monomers and/or polymers was studied by Chinnaswamy and Hanna (Starch/Starke, 43 (10), p. 396 (1991)).
Low-cost production and processing methods for biodegradable plastics are of great importance since they enhance the commercial viability and cost-competitiveness of these materials. Reactive extrusion is an attractive route for the polymerization of a cyclic ester monomer, without solvents, to produce high molecular weight biodegradable plastics. There is thus a need for an improved bulk polymerization process for economically producing aliphatic ester polymers. Compositions of such polymers that possess improved properties and processability for use as biodegradable articles, especially films, are also of greater relevance.